You can earn them through: activity (XP level increasing), showing up daily, playing rated games, winning rated wagered games consistently (eg. play much weaker opponents so you win more than 2/3rds of your games with a decent wager in place, not a really low wager), and entering tournaments (some of those are free) with a chance of winning cubits for prizes.

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ps. If you insist on buying free cubits, there is a green BUY link visible in game mode (top center of screen) that ought to do the trick. If that's not working for you, inquire at one of the Help chat channels in game mode. You can also visit the Store while in game mode (the shopping basket link on the lower left), and choose ADD CUBITS.